The present invention relates to an interferometric sensor and, more particularly, to such a sensor which combines the features of two different configurations of interferometer in order to provide a measuring instrument which has relatively high resolution over an improved unambiguous dynamic range.
A conventional interferometric sensor, for example, embodying a Michelson, Fabry-Perot or Mach-Zehnder configuration, in which a measurand is detected by the change in optical path length induced in a sensing element, has been shown to be applicable to a wide variety of measurands, such as, temperature, acceleration, magnetic field, fluid flow and acoustic pressure, and extremely high resolution has been achieved. However, because the interferometer output is periodic, the value of the measurand derived is ambiguous and the inherent high resolution results in a very limited unambiguous dynamic range. Signal processing schemes, including active phase compensation and heterodyne schemes, have been developed which overcome problems of signal fading and extend the dynamic range. However, these schemes do not provide the absolute value of the measurand when the system is initialised.
More recently, optical fibre polarimetric sensors, which utilise the measurand-induced change in modal birefringence of highly birefringent optical fibre, have been proposed. These polarimetric sensors have a much reduced resolution with a corresponding increase in the period of the interferometer transfer function and they therefore provide an increased unambiguous dynamic range at the expense of reduced resolution. Generally, the ratio of sensitivities for conventional and polarimetric configurations is different for different measurands.